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Teens With 'Normal Weight' Have Eating Disorders, Too: How BMI Fails Them

Eating disorders can be dangerous and life-threatening, but new findings published in the journal Pediatricsreveal drastic weight loss can carry the same health dangers. Australian researchers discovered teens don’t have to be underweight or considered anorexic to have eating disorders, and, in fact, many of them have normal weights, which pulls the controversy of BMI equations under the spotlight to scrutinize its efficacy.

"Emaciated bodies are the typical image portrayed in the media of patients with restricting eating disorders such as anorexia nervosa," lead researcher Melissa Whitelaw, a clinical specialist dietitian at The Royal Children's Hospital in Melbourne, Australia, told Everyday Health. "This paper highlights that it is not so much about the weight but the weight loss that can lead to a serious eating disorder. The complications of malnutrition can occur at any weight."

Whitelaw and her team studied 99 teens between the ages of 12 to 19 in 2005 and found only eight percent of the patients had an eating disorder, but when they checked again four years later, the number jumped to 47 percent. Almost 90 percent of the teens were girls, and those with anorexia nervosa lost approximately 28 pounds, while those with an unspecified eating disorder lost 29 pounds. "I was surprised to see how much it increased," Whitelaw said. "I was also surprised at how similar they were not only physically but also psychologically. Everything about them was anorexia except that they don't look really skinny."

Anorexia is excessive weight loss caused by mental illness and carries with it many psychological symptoms of distorted self-image, depression, anxiety, and fear of weight gain, which is one of the reasons doctors are using BMI as an indicator for eating disorders. But the flaw of the BMI index is that it doesn’t account for the fact muscle weighs more than fat, or that someone could just be abnormally tall and not overweight. When a person develops a body image disorder, they begin to overanalyze and criticize their body in an unhealthy way, and BMI reinforces that problem by categorizing a person with inacurate standards of measurement. Uninformed teens may take the information and use it to confirm their fears of being overweight or higher up on the normal weight scale and drive their number down with excessive and unnecessary weight loss.

Doctors need to look at a person’s body fat content with a more accurate tool, such as water displacement or with the use of calipers that pull skin and fat from the waist, shoulder blade, biceps, and triceps that are added up and plotted on a chart that takes the patient’s sex, age, and measurements into account. Researchers tried to create a new calculation to replace BMI, called the body adiposity index (BAI), which is measured with hip circumference divided by height. However, differences in male and female hip ratio made the calculation fail, according to a 2012 study in the journal Obesity.

In 1832, mathematician Adolphe Quetelet came up with the body mass index equation as a way to categorize a person within the range of underweight, normal, and overweight, using weight equals height squared. At first, researchers used the equation to study population health, but it was quickly adopted by doctors to measure their patients’ body fat percentage, and by 1985 the National Institutes of Health added obesity into the equation. Today, the BMI question is still used as a way to uniformly measure a person’s body fat content, but it begs the question: is it too antiquated to still rely on?

"We are conditioned to think that the key feature of anorexia nervosa is low body mass index,” Cynthia Bulik, director of the Center of Excellence for Eating Disorders at the University of North Carolina at Chapel Hill, told Everyday Health. "In fact, we miss a lot of eating disorders when focusing primarily on weight.”